In the art of hydraulic fluid control cylinders or units for use in a prosthetic leg at the knee joint, for example, as disclosed in U.S. Pat. No. 2,859,451 assigned to the Assignee of the present invention, it is desirable to minimize the size and weight of the unit while providing variable and adjustable damping forces or moments during flexion and extension of the knee and also variable biasing forces for moving the leg to its extended position. As disclosed in the above mentioned U.S. Pat. No. 2,859,451, the hydraulic control unit generally includes a housing which defines an oil reservoir, and a cylinder projects downwardly into the housing in spaced relation. The cylinder confines a control bushing which defines a staggered array of ports connected by corresponding axially extending channels to tapered passages or gaps at opposite ends of the bushing for controlling the oil flow when a piston and piston rod move axially within the bushing to produce variable damping forces. The bushing is axially adjustable within the cylinder for independently adjusting the tapered gaps according to the constant damping forces desired.
A hydraulic fluid control unit of the type disclosed in U.S. Pat. No. 2,859,451 has also been constructed by Applicants' Assignee with a coil compression spring located within the housing and surrounding the cylinder for engaging an annular seal and piston confined between the cylinder and housing. This spring and annular piston form an oil accumulator which receives the displaced hydraulic fluid when the piston rod is forced into the cylinder and housing. The spring biased annular piston also produce an increasing biasing force against the oil as the knee flexes and functions to urge the piston rod outwardly for returning the artificial leg to its extended or generally straight position. Other forms of hydraulic fluid control units for prosthetic legs are disclosed in U.S. Pat. Nos. 3,316,558, 4,065,815, 4,212,087, 4,578,082 and 4,595,179.